The present invention relates to an automatic gain control amplifier suitable for use in a radio transmitter-receiver such as, for example, a portable telephone.
In recent years, there has been a demand for a compactness, handiness, reduction in current consumption of the radio transmitter-receiver, such as a portable telephone. To meet this demand, it has been necessary to integrate a high-frequency circuit used in the portable telephone, which has been formed with individual parts.
To realize the integration of a high-frequency circuit, there has been proposed, for example, an automatic gain control amplifier (AGC amplifier) shown in FIG. 9. In FIG. 9, transistors Q.sub.1 and Q.sub.2 are differentially connected to each other.
More specifically, a collector of the transistor Q.sub.1 is connected to a voltage source V.sub.CC via a resistor R.sub.Cl, a collector of the transistor Q.sub.2 is connected to the voltage source V.sub.CC via a resistor R.sub.C2, and emitters of the transistors Q.sub.1 and Q.sub.2 are connected to a constant current source CC.
The voltage source V.sub.GC served to control a gain is connected to a base of the transistor Q.sub.2, and a grounded capacitor C.sub.B is connected to the base of the transistor Q.sub.2.
An input signal S.sub.IN is inputted to a base of the transistor Q.sub.1 so that an amplified output S.sub.OUT is outputted from the side of the collector of the transistor Q.sub.1. With the above arrangement, the AGC amplifier shown in FIG. 9 operates in a way that the input signal S.sub.IN is inputted to the base of the transistor Q.sub.1, and a voltage of the voltage source V.sub.GC (occasionally referred as V.sub.GC for convenience) is varied to change a gain of the output signal S.sub.OUT outputted from the side of the collector of the transistor Q.sub.1.
In the above AGC amplifier, a saturation power changes since a current I.sub.C1 flowing through the transistor Q.sub.1 changes during the operation. For instance, when the current I.sub.Cl through the transistor Q.sub.1 is decreased by controlling the voltage V.sub.GC, in other words, when the gain is decreased, the saturation power is decreased.
When an output power is changed at a constant input power, for example, a distortion becomes larger when the gain is decreased. To avoid this, it is necessary to increase the current wastefully even when the gain is large, which results in a hindrance to a small current consumption.